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Final Angular Velocity

  1. Nov 11, 2014 #1
    1. The problem statement, all variables and given/known data
    Stacy is throwing a discus. During the throw, she applies an average torque of 90Nm to the discus for 0.3 seconds. The discus has a mass of 1.0kg, and has a radius of gyration of 0.1m about its spin axis. If the initial angular velocity of the discus was zero, what was the angular velocity of the discus at the end of the throw?

    2. Relevant equations
    Wish I learned enough to tell you. Angular acceleration? wf-wi/ delta t. Or angular velocity? Anglef-Anglei/delta t. Or angular momentum? Moment of intertia x angular velocity. Or radius of gyration? Mass x radius of gyration^2

    3. The attempt at a solution
    I wrote all of the key numbers, and wrote the equations for all of the above to see where I could plug in the values. I attempted angular acceleration, but I don't have initial angular velocity or acceleration, I tried radius of gyration and got I = 1.0kg x 0.1m^2 = 0.01kgm^2, but I don't know if that's correct or if it should be 0.01, or if o shouldn't have even bothered with that equation. I went to plug it into the angular momentum equation, but realized I have only the initial angular velocity of 0, so Im just lost and don't know what equation I'm even supposed to be using or how simple or complicated it's supposed to be.
    Last edited: Nov 11, 2014
  2. jcsd
  3. Nov 11, 2014 #2


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    Hello drowsy, welcome to PF :)

    An impressive collection of worded terms. All you have to do is hook them up in the right way and you're in business!
    Did you notice a kind of analogy between angular motion and the usual linear motion ? If not, google...

    The one variable from the problem statement I miss in your collection is torque. Definitely needed, so find out what it is and find some equation to link it to one of the three: the angular displacement, angular velocity or angular acceleration. A link with angular momentum would be a good alternative. Look at the dimensions and glance at the equations you would use in case this would have been linear motion.
  4. Nov 12, 2014 #3
    I'm still pretty lost as someone who had a 66 on an exam for the Linear aspects. None of this has jived with me or my class in two months.
    It says the average torque is 90Nm, but I don't know what to do with that or where to put it. Most equations call for angles, but I can't even fathom of there are angles to be found or how to find them. Im just not Seeing this problem the right way at all for like 24 hours now.
  5. Nov 12, 2014 #4


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    You got the moment of inertia of the discus correct. If you apply a torque T to an object of moment of inertia I, what is the result?
  6. Nov 12, 2014 #5
    You find angular acceleration? Maybe?
    That's the only correlation I keep finding. When I throw them in the T = I x α I get 90Nm / 0.01 = 9,000. Then I do 9,000 / 0.3s = 30,000 to try to get ωf but that seems huge and not correct.
  7. Nov 12, 2014 #6


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    What is your basis for dividing torque by time? Can you quote a relevant equation? What would be the units of the result?
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